THE CONDITIONS OF METAMORPHISM OF A GROSSULAR-WOLLASTONITE VESUVIANITE SKARN FROM THE OMEY GRANITE, CONNEMARA, WESTERN IRELAND, WITH SPECIAL REFERENCE TO THE CHEMISTRY OF VESUVIANITE

The Fountain Hill skarn, which was produced by thermal metamorphism of impure limestone in the Omey Granite aureole, consists of wollastonit e, calcite, grossular-andradite garnet, diopside, B-free vesuvianite a nd small amounts of albite, K-feldspar and quartz. Zoning in vesuviani te is, overall, independent of birefringence but oscillatory concentri c zoning in the mineral is controlled mainly by Ti. In addition to ext ensive within-site substitutions (e.g. F for OH in the OH-sites, Fe2for Mg, Fe3+ for Al, Ti for Al etc. in the Y-sites), there are signifi cant cross-site combined substitutions involving Y-X and Y-Z but not X -Z sites so that ideal solution models for this mineral are not applic able. The thermodynamic mole fraction of Hoisch (1985) is modified to account for the excess of the Sigma Y cations in the Y sites and can b e applied to both B-bearing and B-free vesuvianites. Using the thermod ynamic dataset of Holland and Powell (1990) and taking into account th e existence of andalusite, sillimanite and corundum in associated peli tes, leads to the conclusion that the metamorphic conditions were abou t 640 +/- 20 degrees C and 3.3 +/- 0.3 kb at 0.15 +/- 0.05 XCO(2).